1. Field of the Invention
This invention relates to a liquid crystal display with a liquid crystal layer enclosed by two parallel carrier plates which carry on their facing surfaces electrically conductive coatings as electrodes and a system of dielectric layers.
2. Description of the Prior Art
German Published Non-Prosecuted Application No. DE-OS 28 52 395 describes such a display.
The two plates of a liquid-crystal cell normally support a system of several stacked-up films of different optical density. Such a multiple layer reflects part of the incident light which, if it is not suppressed, is noted as an optical disturbance and decreases the contrast ratio. It has been found in practice that it is not easy to reduce the reflection of the display in an acceptable manner. The reasons for this are, in particular, difficulties in making the display area with low reflection over the electrodes as well as in the remaining area, and this problem must be solved with the least possible expenditure, since liquid-crystal displays have become mass-produced articles and their production has come under considerable cost pressure.
The Offenlegungsschrift No. 28 52 395 cited above describes a type of cell, in which the two substrates carry on top of the electrodes at least one inorganic film. Each film has an index of refraction of between 1.5 and 2.2 at a wavelength of 590 nm, has a thickness of between 50 nm and 200 nm and otherwise must be optimized by means of the Fresnel formulas. Such coating is rather attractive from a cost aspect, since the electrode must normally be coated with an electrically insulating as well as with an orientation layer. However, it has been found that the mentioned material information is still too general; it is not sufficient to eliminate cell reflections in the case of any desired given material and thickness values for the substrate, the conductive layer and the liquid crystal without further measures. Particularly confusing is the situation if the display operates with unpolarized light instead of polarized light, because the birefringent liquid-crystal layer has different indices of refraction for differently polarized rays, a circumstance which has not been taken into consideration in the formerly used optimizing calculations.
In the Japanese patent application No. 53-48 541, an interference-suppressed optical liquid crystal display is described, in which the transparent electrodes have an optical thickness of about .lambda./2 or .lambda., are optically denser than their substrate and are coated with an SiO.sub.2 -layer which in turn is coated with an SiO orientation layer. With this kind of coating, which likewise does not call for additional antireflection coating, acceptable results are obtained only if the prescribed electrode thickness is kept precisely, and even then a trace of color is sometimes still noticeable.